Are Chinese mitten crabs feeding on native vegetation?

The number of Chinese mitten crabs that populates Flemish rivers has strongly risen in the last decade. Recent catches by water managers indicate that over 20000 individuals can populate one single river. There are indications that these large crab populations are feeding on the local aquatic vegetation. However, scientific proof for these indications is currently lacking. It is crucial however to further investigate this: native aquatic vegetation has an important habitat function in our rivers, and helps to attain the water quality standards set by the EU Water Framework directive. To unravel this mystery, Jonas Schoelynck (Global Change Ecology Centre, research group Ecosystem Management) and his colleagues have set up a new experiment.

The experiment, which was just finalized, was executed in the Mesodrome. The Mesodrome is a new research facility at campus Drie Eiken of the University of Antwerp. In this experimental infrastructure, aquatic vegetation was grown under multiple conditions in 12 ponds. Some ponds were covered, to establish light stress in the plants (analogous to turbid water), some ponds received a sub-lethal dose of glyphosate (the active substance in multiple herbicides, receiving large media attention because of a potential future ban) and another pollutant, EDTA. The last ponds received unpolluted water. In all the ponds, Chinese mitten crabs were added in multiple densities. During the experiment, the efficiency of the crabs in attacking the aquatic vegetation was monitored in detail. The first results indicate substantial foraging of the carbs on the water plants, with pollution increasing the rate of plant destruction. The research thus clearly show that crabs can have a substantial impact on the native flora. The results are now written out in detail in a scientific publication.

Who is the Chinese mitten crab?

The Chinese mitten crab (Eriocheir sinensis) is a freshwater crab from the Grapsidae family. It is an invasive crab species, that originates in Asia and made its original appearance in Flemish rivers around 1920. It was imported in the ballast water of ships. The body of the crab can grow up to 7cm width. The crabs are now the largest invertebrate animal species in Flemish rivers and lakes. During the day, the crabs hide in holes or below stones. At night, they get out of their shelters and forage on litter, plants and dead fish. The crabs spend most of their life in freshwater. As adults, they return to the sea to breed. The young animals then return to the freshwater. It is during these migrations that he crabs are mostly seen by people.

The clipping force of the crabs was measured to investigate whether they can exert enough power to cut the vegetation. Some crabs reached forces up to 45 Newton, which equals a weight of 4.5 kg.

The Mesocosm experiment. In the front are the ponds with limited light. The experiment was conducted in the Mesodrome.

Delivery of the crabs to the Mesodrome. The delivery represents only a small fraction of the daily catch by the water managers.

A Chinese mitten crab. The red dot on the back is a marking that allows to recognize the crabs in the experiment.